Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 821: Which vitamin is essential for the hydroxylation of proline and lysine during collagen synthesis?

A. Vitamin B6 (Pyridoxal phosphate)
B. Vitamin C (Ascorbic acid) (Correct Answer)
C. Vitamin B12 (Methylcobalamin)
D. Vitamin B1 (Thiamine)

Explanation: ***Vitamin C (Ascorbic acid)*** - **Vitamin C** acts as a **cofactor** for the enzymes **prolyl hydroxylase** and **lysyl hydroxylase**, which are essential for the hydroxylation of proline and lysine residues. - This **hydroxylation** process is crucial for the formation of stable **collagen triple helices**, which provide structural integrity to connective tissues. *Vitamin B6 (Pyridoxal phosphate)* - **Vitamin B6** is a cofactor for many **enzymes involved in amino acid metabolism**, including transamination, decarboxylation, and racemization. - It does not play a direct role in the **hydroxylation of proline and lysine** during collagen synthesis. *Vitamin B12 (Methylcobalamin)* - **Vitamin B12** is essential for two enzymatic reactions in humans: the conversion of **methylmalonyl-CoA to succinyl-CoA** and the methylation of **homocysteine to methionine**. - It is not involved in the **post-translational modification** of collagen. *Vitamin B1 (Thiamine)* - **Vitamin B1** is a crucial cofactor for enzymes involved in **carbohydrate metabolism**, such as pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase. - It has no known role in the **synthesis or modification of collagen**.

Question 822: Which of the following is NOT a characteristic of biotin deficiency?

A. Nausea
B. Dermatitis
C. Muscular pain
D. Anaemia (Correct Answer)

Explanation: ***Anaemia*** - **Anaemia** is typically *not* a direct characteristic or common symptom of **biotin deficiency**. - While micronutrient deficiencies can sometimes have widespread effects, **biotin's primary roles** do not directly link it to red blood cell production in a way that causes anaemia. *Muscular pain* - **Muscular pain** (myalgia) can be a symptom of **biotin deficiency**, reflecting its role in energy metabolism and mitochondrial function. - Biotin is crucial for **carboxylase enzymes**, which are involved in fatty acid synthesis and gluconeogenesis, processes vital for muscle function. *Nausea* - **Gastrointestinal symptoms** such as nausea can occur in instances of **biotin deficiency**. - This is likely due to the widespread role of biotin in cellular metabolism, affecting various organ systems. *Dermatitis* - **Dermatitis**, often characterized by scaly, reddened skin, is a classic and common symptom of **biotin deficiency**. - Biotin is essential for healthy skin and hair, and its deficiency disrupts these processes.

Question 823: The oxidation of retinol produces:

A. Beta-carotene
B. Retinal (Correct Answer)
C. Retinoic acid
D. Retinine

Explanation: ***Retinal*** - **Retinol** (Vitamin A alcohol) can be oxidized to **retinal** (Vitamin A aldehyde) in a reversible reaction catalyzed by **alcohol dehydrogenase**. - **Retinal** is crucial for vision, where it acts as the chromophore in **rhodopsin**, a light-sensitive pigment found in the photoreceptor cells of the retina. *Beta-carotene* - **Beta-carotene** is a **precursor to retinol**, not a product of its oxidation. It is cleaved to form two molecules of retinal, which can then be reduced to retinol. - It is a **provitamin A carotenoid** found in plants, which is converted into vitamin A in the body. *Retinoic acid* - **Retinoic acid** is produced by the **irreversible oxidation of retinal**, not directly from retinol. This conversion is catalyzed by **retinal dehydrogenase**. - **Retinoic acid** is essential for gene transcription, cell differentiation, growth, and immune function, acting as a hormone. *Retinine* - The term **"retinine"** is an outdated and less commonly used term for **retinal** or **retinene**. Modern biochemical nomenclature standardizes the term as retinal. - While historically used, it does not represent a distinct chemical entity separate from **retinal** in the context of vitamin A metabolism.

Question 824: Which vitamin can be synthesized from tryptophan?

A. Niacin (Correct Answer)
B. Riboflavin
C. Cobalamin
D. Folic acid

Explanation: ***Niacin*** - **Niacin** (vitamin B3) can be endogenously synthesized in the human body from the essential amino acid **tryptophan**. - Approximately **60 mg of tryptophan** is required to synthesize 1 mg of niacin. *Riboflavin* - **Riboflavin** (vitamin B2) cannot be synthesized from tryptophan; it is obtained solely through dietary sources. - It plays a crucial role as a **coenzyme** in numerous metabolic reactions, including energy production. *Cobalamin* - **Cobalamin** (vitamin B12) cannot be synthesized by humans and must be obtained from animal products or supplements. - Its synthesis is complex and primarily occurs in **microorganisms**. *Folic acid* - **Folic acid** (vitamin B9) cannot be synthesized from tryptophan; it is primarily consumed through diet. - It is critical for **DNA synthesis** and repair, and cell division.

Question 825: Carboxylation of clotting factors by vitamin K is required for them to be biologically active. Which amino acid is carboxylated?

A. Histidine
B. Serine
C. Glutamate (Correct Answer)
D. Aspartate

Explanation: ***Glutamate*** - **Vitamin K** acts as a cofactor for the enzyme **gamma-glutamyl carboxylase**, which carboxylates specific **glutamate residues** in clotting factors (II, VII, IX, and X). - This carboxylation forms **gamma-carboxyglutamate (Gla)** residues, which allows the clotting factors to bind **calcium ions** and thereby to phospholipid surfaces, enabling the coagulation cascade. *Histidine* - **Histidine** is an amino acid that plays a role in enzyme active sites and metal ion chelation, but it is not the target for **vitamin K-dependent carboxylation**. - Its imidazole ring can donate and accept protons, giving it a role in **pH buffering** and catalytic mechanisms. *Serine* - **Serine** is an amino acid that undergoes post-translational modifications such as **phosphorylation** (by kinases) and **O-glycosylation**. - However, it is not the amino acid specifically targeted by **vitamin K-dependent carboxylation** for the activation of clotting factors. *Aspartate* - **Aspartate** is an acidic amino acid that can bind **metal ions** and participates in various metabolic pathways and enzyme active sites. - While structurally similar to glutamate, it is not the amino acid specifically targeted by **vitamin K-dependent carboxylation** for the activation of clotting factors.

Question 826: Thiamine pyrophosphate (TPP) plays a crucial role as a cofactor in multiple enzymatic reactions. Which of the following represents its PRIMARY role in linking glycolysis to the citric acid cycle for energy production?

A. It is required for the conversion of alpha-ketoglutarate to succinyl-CoA in the Krebs cycle.
B. It is involved in the synthesis of neurotransmitters.
C. It is a co-factor for the enzyme that converts pyruvate to acetyl-CoA. (Correct Answer)
D. It is essential for the transketolase reaction in the pentose phosphate pathway.

Explanation: ***It is a co-factor for the enzyme that converts pyruvate to acetyl-CoA.*** - **Thiamine pyrophosphate (TPP)**, the active form of thiamine, is a critical cofactor for the **pyruvate dehydrogenase complex**, which catalyzes the conversion of **pyruvate to acetyl-CoA**. - This reaction is the **primary link** between glycolysis and the citric acid cycle (Krebs cycle), making it essential for aerobic glucose metabolism and ATP production. - TPP deficiency impairs this step, leading to lactate accumulation and neurological complications seen in beriberi and Wernicke-Korsakoff syndrome. *It is required for the conversion of alpha-ketoglutarate to succinyl-CoA in the Krebs cycle.* - TPP is indeed a cofactor for the **α-ketoglutarate dehydrogenase complex** along with lipoic acid, CoA, FAD, and NAD+. - While this is an important energy-producing step **within** the citric acid cycle, the question asks about the primary link between glycolysis and the cycle. - This reaction occurs after pyruvate has already entered the mitochondrial pathways. *It is involved in the synthesis of neurotransmitters.* - Thiamine deficiency can indirectly affect neurotransmitter function due to impaired energy metabolism in neurons. - However, TPP is **not a direct cofactor** for enzymes involved in the synthesis of major neurotransmitters like acetylcholine, dopamine, or serotonin. - Its neurological importance stems primarily from its role in glucose metabolism and oxidative processes. *It is essential for the transketolase reaction in the pentose phosphate pathway.* - TPP is indeed the essential cofactor for **transketolase** in the **pentose phosphate pathway (PPP)**. - The PPP generates NADPH (for biosynthesis and antioxidant defense) and ribose-5-phosphate (for nucleotide synthesis). - While metabolically important, this pathway is primarily anabolic rather than directly involved in ATP production through oxidative phosphorylation, which is the main energy production pathway.

Question 827: Niacin is synthesized from:

A. Phenylalanine
B. Tryptophan (Correct Answer)
C. Tyrosine
D. Methionine

Explanation: ***Tryptophan*** - **Niacin (Vitamin B3)** can be synthesized endogenously from the essential amino acid **tryptophan**. - This conversion is a multi-step pathway, requiring **cofactors** like **vitamin B6**, **riboflavin**, and **iron**. *Phenylalanine* - **Phenylalanine** is an essential amino acid, but it is primarily a precursor for **tyrosine**, not niacin. - Its metabolic pathway leads to the synthesis of neurotransmitters like **dopamine** and **norepinephrine**. *Tyrosine* - **Tyrosine** is a non-essential amino acid synthesized from phenylalanine. - It is a precursor for **thyroid hormones**, **catecholamines** (dopamine, norepinephrine, epinephrine), and **melanin**. *Methionine* - **Methionine** is an essential amino acid primarily involved in **methylation reactions** and the synthesis of **cysteine**. - It is not a direct precursor for niacin synthesis.

Question 828: A patient is found to have sun-sensitive dermatitis, diarrhea, and dementia. He must be suffering from the deficiency of which of the following?

A. Iron
B. Riboflavin
C. Niacin (Correct Answer)
D. Folic acid

Explanation: ***Niacin*** - The combination of **sun-sensitive dermatitis**, **diarrhea**, and **dementia** is the classic triad of symptoms for **pellagra**, a deficiency of **niacin (vitamin B3)**. - Niacin is crucial for **NAD+** and **NADP+** synthesis, which are essential coenzymes in over 400 enzymatic reactions, particularly in energy metabolism and DNA repair. *Riboflavin* - Deficiency of **riboflavin (vitamin B2)** typically presents with **cheilosis**, **angular stomatitis**, and **glossitis**, not the dermatological, gastrointestinal, and neurological issues described. - **Ariboflavinosis** can also cause oily, scaly rashes and light sensitivity, but not the pellagra triad. *Iron* - **Iron deficiency** leads to **anemia**, characterized by fatigue, pallor, and weakness. - While it can affect cognitive function (e.g., **"brain fog"** and **pica**), it does not cause the specific skin rash or chronic diarrhea associated with pellagra. *Folic acid* - **Folic acid deficiency** primarily causes **macrocytic anemia** and can lead to **neural tube defects** in developing fetuses. - Neurological symptoms like depression and cognitive impairment can occur, but it does not cause the characteristic **dermatitis** or **diarrhea** seen in pellagra.

Question 829: Thiamine deficiency is best diagnosed by?

A. Blood thiamine level
B. Transketolase activity in blood (Correct Answer)
C. Aldolase activity in blood
D. Urinary thiamine level

Explanation: ***Transketolase activity in blood*** - **Transketolase** is a thiamine pyrophosphate (TPP)-dependent enzyme, and its activity in red blood cells is considered the **gold standard** for assessing thiamine status. - A **decrease in transketolase activity** that improves after the addition of TPP in vitro (TPP effect) is highly indicative of thiamine deficiency. *Blood thiamine level* - While a direct measure, **blood thiamine levels** can be influenced by recent dietary intake and may not accurately reflect the body's overall thiamine stores or functional deficiency. - Furthermore, measuring total blood thiamine doesn't always correlate well with the **functional status** of thiamine-dependent enzymes. *Aldolase activity in blood* - **Aldolase** is an enzyme involved in glycolysis, but its activity is **not dependent on thiamine**. - Therefore, measuring aldolase activity provides no information regarding thiamine status. *Urinary thiamine level* - **Urinary thiamine levels** primarily reflect recent thiamine intake and renal excretion rather than the body's total thiamine stores or a functional deficiency. - Low urinary thiamine can suggest deficiency, but it's **less reliable** than functional assays.

Question 830: The highest concentration of Vitamin A is seen in which of the following sources?

A. Shark liver oil
B. Papaya
C. Polar bear liver (Correct Answer)
D. Cod liver oil

Explanation: ***Polar bear liver*** - Polar bears consume a diet rich in **fatty fish and seals**, which contain high levels of **retinol**, leading to extreme accumulation in their livers. - Ingesting polar bear liver can be **toxic to humans** due to the excessively high Vitamin A content, causing hypervitaminosis A. *Cod liver oil* - This is a well-known source of **vitamins A and D**, but its concentration is significantly lower than that found in the liver of apex predators like polar bears. - While beneficial as a supplement, it does not pose the immediate threat of toxicity seen with polar bear liver consumption. *Shark liver oil* - Primarily known for its high content of **squalene** and **alkylglycerols**, which are thought to have immune-boosting properties. - It contains Vitamin A but in much lower concentrations compared to polar bear liver and is not its primary claim to fame. *Papaya* - Papaya is a fruit rich in **beta-carotene**, a precursor to Vitamin A. - While a healthy source of pro-vitamin A, its concentration is significantly lower and in a different form (provitamin A) compared to the pre-formed retinol found in animal livers.

Practice by Chapter

Fat-Soluble Vitamins: A, D, E, K

Practice Questions

Vitamin A and Vision

Practice Questions

Vitamin D and Calcium Metabolism

Practice Questions

Vitamin E and Antioxidant Functions

Practice Questions

Vitamin K and Blood Coagulation

Practice Questions

Water-Soluble Vitamins: B Complex and C

Practice Questions

Thiamine (B1) and Pyruvate Dehydrogenase

Practice Questions

Riboflavin (B2) and Flavin Coenzymes

Practice Questions

Niacin and NAD/NADP

Practice Questions

Vitamin B6 and Transamination

Practice Questions

Folate and Vitamin B12 in One-Carbon Metabolism

Practice Questions

Vitamin C and Collagen Synthesis

Practice Questions

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